A fiber laser device of fabry-perot type is now in practical use as a type of fiber laser device. In this fiber laser device of fabry-perot type, pumping light of a predetermined wavelength is input to a rare earth-doped fiber from an pumping light source and light of longer wavelength than the input pumping light is emitted from the rare earth-doped fiber. Then, the light emitted from the rare earth-doped fiber is reflected by one of FBGs (Fiber Bragg Grating) formed on both sides of the rare earth-doped fiber. The light that is reflected is again input to the rare earth-doped fiber and amplified. The amplified light is then reflected by the FBG on a side of the rare earth-doped fiber, which is opposite to the FBG that has reflected the light. In this manner, light is amplified each time when the light is reflected by the FBGs formed on both sides of the rare earth-doped fiber and passes through the rare earth-doped fiber and a part of the amplified light is output through one of the FBGs as laser light.
Patent Document 1 listed below discloses an example of a fiber laser device. In the fiber laser device disclosed in Patent Document 1, two FBGs have their center wavelengths of light reflected by the FBGs, which are Bragg wavelengths, same as each other. Accordingly, light of the wavelength same as the Bragg wavelength of each of the FBGs resonates between the two FBGs and is output through one of the FBGs as laser light.    [Patent Document 1] Japanese Patent No. 3219415